1. Field
The present invention relates to an input circuit and method and, more particularly, to an input circuit with a variable reference level generator that varies a reference signal level responsive to an input signal and an associated method.
2. Related Art
An input circuit determines a signal level of an externally provided input signal. Typically, the input circuit compares the reference signal level with the input signal level to determine whether the input signal level is higher or lower than the reference signal level.
Often, the input signal is distorted by the signal line through which it is input, and this distortion increases as the semiconductor device's operation speed increases. The faster the semiconductor device operates, i.e., the higher the frequency of the input signal, the farther from full swing the input signal deviates, and thus the poorer the quality of the input signal.
FIGS. 1A and 1B are block diagrams of a conventional input circuit. FIG. 1A shows a case where the input circuit determines the input signal level using an internally generated reference level. FIG. 1B shows a case where the input circuit determines the input signal level using an externally generated reference level.
The circuits shown in FIGS. 1A and 1B operate as follows.
In FIG. 1A, a reference level generator 100 outputs a reference signal FLREF with a fixed reference level. In FIGS. 1A and 1B, an input unit 200 compares the fixed level reference signal FLREF (FIG. 1A), or an externally generated fixed level reference signal FLREF (FIG. 1B), with an externally input signal IN, determines whether the input signal IN is at a logic low or high level, and outputs an output signal OUT based on the result.
FIGS. 2A and 2B are signal waveform diagrams illustrating the operation of the input circuits shown in FIGS. 1A and 1B. FIG. 2A is a waveform diagram of an original input signal IN-O. FIG. 2B is a waveform diagram of an input signal IN seen from the input unit 200. In FIG. 2B, the fixed reference signal is denoted FLREF.
As the frequency of the original input signal IN-O increases, loading effect of the signal line increases and thus the input signal becomes incapable of a full swing. The input signal IN observed from the input unit 200 appears as a distorted version of the original input signal IN-O, as shown in FIG. 2B.
And the level of the reference signal FLREF is fixed as shown in FIG. 2B. The reference signal FLREF may be generated by the internal reference level generator 100 and input to the input unit 200 or applied externally to the input unit 200.
The input unit 200 compares the level of the reference signal FLREF with the level of the input signal IN and determines whether the input signal IN is at a logic low or high level.
In a typical input circuit, since the level of the reference signal FLREF is fixed, when a short pulse is generated in the original input signal IN-O, it is difficult to determine whether the input signal IN is at a logic high or low level.
Specifically, as shown in FIG. 2B, when the input signal IN is maintained at a low level (or a high level) for a predetermined time interval and then instantaneously changed to a high level (or a low level) at t1, t2, and t3, a difference in level between the input signal IN and the reference signal FLREF is remarkably smaller than at other times.
Put differently, when the original input signal IN-O is maintained at a low level (or a high level) for a predetermined time interval, the input signal IN has a level almost equal to the low level (or the high level) at which the original input signal IN-O is maintained. When the original input signal IN-O makes a low-to-high (or high-to-low) transition, the input signal IN cannot reach a sufficiently higher level (or a sufficiently lower level) than the level of the reference signal FLREF at a time where the input unit 200 determines the level of the input signal IN. As a result, a difference in level between the input signal IN and the reference signal FLREF is reduced.
In a typical input circuit, the level of the reference signal FLREF is fixed so that it is very difficult to normally determine the level of the input signal IN in the above-described worst case.